One type of spatial light modulator (SLM) is the digital micromirror device (DMD). Digital micromirror devices have also been called deformable micromirror devices although that term is now used exclusively for devices that operate in an analog mode. DMDs are finding many uses in light deflecting technologies. The devices are small mirrors which rotate or deflect in response to an electrical signal to redirect incident light. In some applications, an array of DMDs is used to modulate light and produce patterns or images when the individual DMD elements, called pixels, are selectively rotated.
DMDs are typically used in a dark field projection arrangement and may be used, for example, in HDTV applications where a large array of pixels is necessary for the desired clarity. The ideal DMD should produce a high resolution image with uniform brightness, high contrast, and be extremely reliable. DMD elements must be designed to operate many times without failure in order to be reliable enough for commercial display systems. This requires a rugged, producible design that minimizes the stress and wear on the DMD elements.
Primarily two types of DMDs are used in imaging applications, the conventional torsion beam DMD and the hidden hinge DMD. Both use electrostatic attraction to rotationally deflect a mirror and twist a pair of torsion hinges. The conventional torsion beam DMD mirror is supported directly by the torsion hinges which are also attached to hinge support posts. One drawback of the conventional torsion beam DMD is the hinges and hinge support posts scatter incident light which reduces the contrast ratio of the display. Also, because the torsion hinges and hinge support posts are located in the plane of the mirrors, the mirror area is reduced, resulting in decreased image brightness. The hidden hinge architecture addresses these problems by moving the hinge support posts and torsion hinges below the mirrors. The mirrors are supported above the plane of the hinges by a mirror support post.